1. Field of the Invention
The present invention relates to backlight circuits, and more particularly to backlight circuits for large size liquid crystal displays (LCDs).
2. Description of the Related Art
LCD panels are used in various applications ranging from portable electronic devices to fixed location units, such as laptop PCs, video cameras, automobile navigation systems, and industrial machines. The LCD panel itself cannot emit light but must be backlit by a light source. The most commonly used backlight source is a cold-cathode fluorescent lamp (CCFL). Usually, igniting and running the CCFL requires a high alternating current (AC) signal. To generate such a high AC signal from a direct current (DC) power source, e.g., a rechargeable battery, a DC/AC inverter is designed to drive the CCFL.
However, in recent years, there has been increasing interest in large size LCD displays, used for example in LCD TV sets and computer monitors, which require multiple CCFLs to provide high intensity illumination. Nowadays, multiple long CCFLs are widely used in large LCD panels. Normally, terminals of the long CCFL are driven by two separate inverters, each of which provides an alternating voltage with a 180-degree phase shift relative to the other. This is a so-called floating lamp structure as illustrated in FIG. 1. An exemplary circuit 100 in FIG. 1 includes inverters 110 and 120 that are coupled respectively to the terminals of a long CCFL 130 and designed to provide alternating voltages +V and −V. The floating lamp structure in FIG. 1 may reduce the leakage current between the long CCFL 130 and a chassis 140, which in turn maintains even brightness over the entire CCFL 130. However, the circuit 100 becomes complicated as the number of CCFLs increases. Additionally, with multiple CCFLs, it is hard to ensure current balance among all CCFLs and the imbalance of the CCFL currents will give rise to a reduced lifetime of the CCFLs and non-uniformity of brightness.
FIG. 2 illustrates a schematic diagram of a conventional multi-CCFL backlight system 200. The multi-CCFL backlight system 200 includes a current balance circuit 210 comprising current balance transformers TC6 to TC10 (TC1 to TC5) that are coupled to the high-voltage terminal of the power transformer T1 (T2). However, the high-voltage terminal of the power transformer T1 (T2) experiences an AC voltage of more than a thousand volts for such applications, and therefore devices coupled to the high-voltage terminal require extra attention to prevent any safety hazards and concerns. Additionally, there are twice as many current balance transformers (TC1 to TC10) as lamps (CCFL1 to CCFL5) in the system, adding to the complexity and overall cost to the backlight system.
Some other configurations of multi-CCFL backlight systems are taught by U.S. Pat. No. 6,781,325 B2 in FIG. 3 and illustrated by conventional circuits in FIGS. 4 and 5. In FIG. 3, an exemplary circuit 300 includes a balance circuit 310 comprising current balance transformers TC10 to TC18 (TC1 to TC9). The balance circuit 310 is designed to balance lamp currents of N CCFLs by using 2N−2 current balance transformers. In FIG. 4, an exemplary circuit 400 includes a balance circuit 410 comprising current balance transformers TC11 to TC20 (TC1 to TC10). The balance circuit 410 is designed to balance lamp currents of N CCFLs by using 2N current balance transformers. In FIG. 5, an exemplary circuit 500 includes a balance circuit 510 comprising current balance transformers TC11 to TC20 (TC1 to TC10). Also, the balance circuit 510 is designed to balance lamp currents of N CCFLs by using 2N current balance transformers. All these configurations of multi-CCFL backlight systems suffer from large element count, complex circuitry and safety limitations as mentioned above.
Furthermore, the lamp array usually deployed in the multi-lamp backlight systems employs long CCFLs, which are expensive and difficult to replace if broken. Additionally, the long CCFLs tend to deposit mercury on the bottom wall and the deposited mercury will eventually degrade the uniformity of illumination. Moreover, the long CCFLs are difficult to manufacture and need large storage space.